1. Field of the Invention
The present invention relates generally to the art of railway signaling. More particularly, the invention relates to a vital apparatus and method for transmitting railway signal information from an electrical link input at a first terminal location to an electrical link output at a second terminal location.
2. Description of the Prior Art
In the control of railroad and rail-borne transit vehicles, control signals are frequently passed over significant distances. These control signals may, for example, actuate switch turnouts to allow traffic flow to branch from one track to another. Additionally, the control signals may actuate wayside indicators to display an appropriate aspect for the prevailing speed conditions.
In order to prevent a system failure from causing a problem, many railway signalling components are designed to have "vital" characteristics. In the art, the term "vital" signifies a component designed to give the most restrictive condition in the event of a failure. It is thus desirable to have signal links for passing signal information between field locations be constructed using vital design principles.
A vital railway signal link practicing the present invention transmits information between an electrical link input at a first terminal location and an electrical link output at a second terminal location. In response to a DC input signal applied to the electrical link input, transmitting means at the first terminal location emit a light signal modulated at a preselected frequency. The light signal is transmitted to receiver means at the second terminal location via an optical conductor such as an optical fiber. A DC output signal is then provided at the electrical link output. To prevent stray ambient light from causing an errant output signal at the electrical link output, discriminator means are provided which assure essentially no output signal if other than a light signal modulated at the preselected frequency appears at the receiver means. Thus, the term "discriminator" is used herein to signify means that are adjusted to accept or reject signals of different characteristics (such as amplitude or frequency).
In presently preferred embodiments, the light signal is produced by application of the DC input signal to a free running oscillator. A periodic electrical signal produced by the oscillator is applied to photo-emission means, such as an infrared light emitting diode driven by a Darlington emitter follower transistor network. At the second terminal location, photo-sensitive input means detect the periodic light signal and produce an analogous electrical signal. The discriminator characteristics are preferably provided by a relatively narrow bandpass filter which may be constructed having a pair of resonant circuits coupled by an electrical isolation coupler. The bandpass filter receives the analogous electrical signal and produces a filtered output signal. Signals of other frequencies are blocked. Output means, which may comprise impedance matching and amplification transistor networks feeding a rectifier network, receive the filtered electrical signal and produce the DC output signal.
Other presently preferred embodiments of the invention are bidirectional, having a receiver and transmitter at both terminal locations. Still other embodiments utilize one or more repeaters to compensate accrued line losses occurring in the conductor. Each repeater may simply comprise a receiver having a DC output tied to the DC input of a transmitter. Using such repeaters, the effective operable length of the railway signal link may be extended to virtually any desired value.